1. Field of the Invention
The present invention relates generally to apparatus and techniques for reversing the effects of inhaled general anesthetics. More particularly, the present invention relates to use of ventilation and rebreathing apparatus and, optionally, respiratory monitoring apparatus, in conjunction with one another to reverse the effects of inhaled general anesthetics.
2. Background of Related Art
General anesthesia is often administered to subjects as surgical procedures are being performed. Typically, a subject who is subject to general anesthesia is “hooked up” to a ventilator by way of a breathing circuit. One or more sensors may communicate with the breathing circuit to facilitate monitoring of the subject's respiration, the anesthesia, and, possibly, the subject's blood gases and blood flow. One or more anesthetic agents are typically administered to the subject through the breathing circuit.
Examples of breathing circuits that are used while anesthesia is being administered to a patient include circular breathing circuits, which are also referred to in the art as “circle systems,” and Mapleson or Bain type breathing circuits, which are also referred to herein as Bain systems for the sake of simplicity.
Circle systems are typically used with adult patients. The expiratory and inspiratory limbs of a breathing circuit of a circle system communicate with one another, with a carbon dioxide remover, such as a soda lime can, being disposed therebetween. As the expiratory and inspiratory limbs communicate with one another, a circle system will typically include two or more sets of one-way valves to prevent a patient from rebreathing just-expired, CO2-rich gases.
Bain systems are typically used with smaller patients (e.g., children). Bain systems include linear tubes through which both inspiratory and expiratory gases flow. Fresh gases are typically directed toward a patient interface to remove the just-expired gases therefrom before the patient can rebreathe them. As long as the fresh gas flow is higher than the flow of the patient's ventilation, there is little or no rebreathing.
When a general anesthesia is administered to a subject, respiratory or inhaled anesthetics are delivered to a patient in low concentrations, typically being diluted to a concentration of about 1% to about 6%, depending on the type of anesthetic agent used. As the subject inhales a general anesthetic agent, the anesthetic agent is carried into the lungs, where it enters the bloodstream, and is carried by the blood to various other body tissues. Once the concentration of the anesthetic reaches a sufficient level, or threshold level, in the brain, which depends upon a variety of subject-specific factors, including the size and weight of the subject, the subject becomes anesthetized. The subject remains anesthetized so long as the concentration of the anesthetic agent in the brain of the subject remains above the threshold level.
Once the procedure, typically surgery, for which the general anesthesia is given, has been completed, it is usually desirable to reverse the effects of the general anesthetic as soon as possible. Reversal of the effects of general anesthesia allows the surgical team to vacate the operating room, thereby freeing it up for subsequent surgeries and possibly reducing the cost of surgery, and also permits the anesthetist to tend to other patients, and conserves the typically expensive anesthetic agents that are used. In addition, for safety reasons, it is desirable to minimize the time a subject is under general anesthesia. Other benefits of quickly reversing anesthesia include better cognitive function for elderly patients immediately following surgery and enabling patients to protect their own airway sooner.
Reversal or discontinuation of the general anesthetic state requires that levels of the anesthetic agent in the brain decrease below the threshold level, or that the anesthetic agent be removed from the subject's brain.
It has long been known that activated charcoal and other substances can be used to selectively adsorb gaseous anesthetic agents. Accordingly, activated charcoal has found conventional use in adsorbers, such as that described in U.S. Pat. No. 5,471,979, issued to Psaros et al., that prevents anesthetic agents from escaping the breathing circuit and entering the operating room. In this regard, activated charcoal adsorbers are typically placed in the exhaust flow of the anesthesia delivery system. The potentially deleterious effects of exhaust anesthetic gases into the operating room are thereby avoided. Further, as most halocarbon anesthetics are considered to be atmospheric pollutants, the charcoals or other adsorbents of conventional anesthetic agent adsorbers prevent pollution that may be caused if gaseous anesthetic agents were otherwise released into the environment.
U.S. Pat. No. 5,094,235, issued to Westenskow et al. (hereinafter “Westenskow”), describes the use of activated charcoal to hasten the removal of gaseous anesthetic agents from breathing circuits. While such a technique would be useful for preventing the reinhalation of previously exhaled anesthetic agents, more could be done to hasten the rate at which anesthetic agents are removed from the subject's brain.
Typically, the rate at which blood flows through the brain and a subject's breathing rate and breathing volume are the primary factors that determine the rate at which the levels of anesthetic agent are removed from the brain of the subject. The rate of blood flow through the brain is a determining factor because the blood carries anesthetic agents away from the brain and to the lungs. The breathing rate and breathing volume are important since they increase the rate at which anesthetic agent may be removed from the blood and transported out of the body through the lungs.
Hyperventilation has been used to increase the breath volume and/or rate of a subject and, thereby, to facilitate the removal of anesthetic agents from the subject's lungs. However, hyperventilation typically results in a reduced level of carbon dioxide (CO2) in blood of the subject (PaCO2). When PaCO2 levels are decreased, the brain is less likely to signal the lungs to breathe on their own and the patient remains dependent on the ventilation from an artificial respirator. See U.S. Pat. No. 5,320,093, issued to Raemer (hereinafter “Raemer”). Additionally, the reduced PaCO2 levels that result from hyperventilation are known to cause a corresponding reduction in the rate at which blood flows through the brain, which actually decreases the rate at which the blood can carry anesthetic agents away from the brain.
Rebreathing processes, in which a subject “rebreathes” previously exhaled, CO2-rich air, have been used to prevent significant decreases in PaCO2 levels during such hyperventilation. The apparatus that have been conventionally used to effect such processes, however, do not filter anesthetic agent from the exhaled air before the subject rebreathes the same. Consequently, the patient also rebreathes the previously exhaled anesthetic agent, which effectively prolongs the process of reversing the general anesthesia.
The computerized system described in Raemer was designed to overcome purported deficiencies with hyperventilation and rebreathing. The system of Raemer infuses CO2 from an external source into the breathing circuit and, thus, into the subject's lungs (i.e., the CO2 is not rebreathed by the subject) as general anesthesia is being reversed to speed the rate of reversal and, thus, recovery of the subject from the general anesthesia. The teachings of Raemer with respect to infusion of CO2 from an external source are limited to avoidance of reintroducing anesthetic agents into the subject's brain while increasing the subject's PaCO2 to a level that will facilitate reinitiation of spontaneous breathing by his or her brain as early as possible. As the technique and system that are taught in Raemer do not include increases in the breathing rate or breathing volume of a subject, they do not accelerate the rate at which a subject recovers from anesthesia.
Accordingly, there are needs for processes and apparatus which increase the rate at which blood carries anesthetic agents from the brain, as well as the rate at which the lungs expel the anesthetic agents from the body in order to minimize the time required to reverse the levels of anesthetic agents in the brain to reverse the effects thereof.